Marine structures

ABSTRACT

A maritime assembly having its foundation raft and tower structure constructed separately, and then joined by the engagement of spigot and socket portions. By this reduction of pressure between those portions, hydrostatic force can be used to secure the tower structure to the foundation raft. Features include the use of a semi submersible to lower the foundation raft to the seabed, and the use of sand jacks to found the raft thereon.

United States Patent Hansen July 29, 1975 [541 MARINE STRUCTURES3,528,254 9/1970 Graham 61/465 3,698,198 10/1972 Phelps 61/465 Inventor:Fmde 1011a" Hansen KmgSWOOd 3,736,756 6/1973 LlOyd 61 /46.5

England [73] Assignee: Redpath Dorman Long (North Sea) FOREIGN PATENTSAPPLICATIONS Limmd, Bedford England 1,088.804 10/1967 Un ted K ngdom61/465 551,449 2/1943 Umted Kmgdom 61/465 [22] Filed: Nov. 29, 1973 [21]Appl. No.: 420,294 Primary Examiner-Jacob Shapiro Attorney, Agent, orFirmBacon & Thomas [30] Foreign Application Priority Data Dec. 1, 1972United Kingdom 55533/72 ABSTRACT July unltcd Kingdom 34679/73 A maritimeassembly having its foundation raft and July unfted 34681/73 towerstructure constructed separately, and then July 20, 1973 Umted Kmgdom34682/73 joined by the engagement of Spigot and socket pop tions. Bythis reduction of pressure between those por- U-S- tions, hydrostaticforce can be used to secure the [51] '3 E02B 17/00 E021) 23/16 towerstructure to the foundation raft. Features in- [58] new Search 53'74clude theuse of a semi submersible to lower the foundation raft to theseabed, and/the use of sand jacks to [56] References C'ted found theraft thereon.

UNITED STATES PATENTS 3.522.709 8/1970 Vilain 61/465 17 27 D'awmg gunsSHEET PATENTEDJULZSISYS' F/ai PATENTEU JUL 2 9 I975 SHEET F/G. 770. A n

PATENTEI] JUL2 91975 SHEET Q GI MARINE STRUCTURES The invention relatesto the construction and founding of maritime assemblies.

It is known to construct platforms founded in water. Such platforms havea foundation in a sub-aqueous bed, a tower structure extending upwardlytherefrom, and a working deck and/or superstructure supported on top ofthe tower structure above the water surface.

The foundation may be formed by driving piles into the sub-aqueous bed.This method requires considerable time and effort on site. and may haveto be interrupted in stormy weather.

To avoid the need for pile driving it has been proposed to foundplatforms deriving stability partly from their own weight. These lastmentioned platforms may be termed gravity structures. One difficulty tobe expected with gravity structures" is that when completed they areunwieldy to tow to their intended sites, and are relatively vulnerableto adverse weather.

The present invention is concerned with a maritime assembly constructedby first sinking a foundation raft to the sub-aqueous bed, and thenfixing an upstanding tower structure thereto. The foundation raft andtower structure can be towed seperately to the site, and foundedconsecutively during relatively short periods of fair weather.Subsequently the deck or super structure can be added above water levelto complete the platform.

The term maritime assembly as used herein means an assembly standing orintended to stand in a sea, lake, estuary or other expanse of water,without being structurally connected to land other than the sub-aqueousbed beneath the expanse of water; and includes the combination of afoundation raft and a tower structure.

The invention provides a maritime assembly for disposal on a subaqueousbed, comprising a foundation raft to be founded on the bed. and a towerstructure to project upwardly therefrom when founded, in which one ofthe foundation raft and the tower structure has a socket portion and theother has a corresponding spigot portion for engagement therewith.

It is preferred that there is means to evacuate a space between thespigot and socket portions, whereby external hydrostatic pressure willkeep the portions in engagement, so fixing the tower structure inposition on the foundation raft.

It is preferred that a seal is carried on a shoulder surrounding thespigot to engage material surrounding the rim of the socket.

Preferably the space can be de-watered, and there is means to grout thespace.

Preferably a hollow tubular member is incorporated in the towerstructure. which member communicates with a valve leading to the space.

In one preferred form the tower structure carries the spigot portion.and the socket is incorporated in the foundation raft.

According to a preferred feature of the invention the foundation raftand the tower structure have shafts which are aligned when the spigotand socket are in engagement. whereby to provide a continuous shaftextending from near the water surface to the subaqueous bed beneath thefoundation raft.

According to another preferred feature of the invention the towerstructure has internal voids to render the tower structure buoyant fortowing to a position above the founded foundation raft, and which may beselectively flooded. whereby the tower structure may be ballasted tosink with its axis substantially vertical.

According to yet another preferred feature of the invention there is asemi-submersible capable of being disposed upon the foundation raft, thefoundation raft is capable of being deliberately flooded at will, andthe semi-submersible has means for lowering the partially floodedfoundation raft to the seabed.

Preferably the semi-submersible has buoyant legs extending from an upperlevel to a lower level, whereby the semi-submersible floats with a smallwaterplane area during the lowering of the foundation raft.

Advantageously there are on the base of the foundation raft three units,each having two vertically spaced chambers, and there are means to causesand or the like to pass out of the lower chambers, whereby the raft canbe lowered towards the seabed by displacement of the sand or the like.

It is preferred that each unit has a plate arranged at the foot of thelower chamber to bear on the seabed, which plate is connected to theunit by cables, and is slideable within the lower chamber.

It is also preferred that the units themselves are moveable as pistonswithin cylinders attached to or forming part of the base of the raft,and the height of the pistons can be adjusted by movement of sand fromthe aforesaid cylinders.

Preferably the pistons are suspended within the cylinders to preventthem from dropping out.

The invention also provides a method of founding a maritime assembly,which includes the steps of founding a foundation raft on a subaqueousbed, and sinking a tower structure onto the foundation raft, in whichspigot and socket portions on the foundation raft and the towerstructure are engaged to secure the tower structure to the raft.

It is preferred that the tower structure is held on the foundation raftby means of a sealed de-watered space between the foundation raft andthe tower structure.

It is preferred that the tower structure is guided towards thefoundation raft by means of cables which pass from the foundation raftthrough sheaves on the tower structure.

It is preferred that a semi-submersible is placed on the foundationraft, the foundation raft and semisubmersible are floated to a site, thefoundation raft is at least partially filled with water, the foundationraft is lowered to the seabed by means of supporting elements suspendedfrom the semi-submersible, the foundation raft is secured on the seabed.and the semisubmersible is removed.

It is preferred that three units are located on the base of thefoundation raft. each of which units have two vertically spacedchambers, the lower of which is separated from the seabed; including thestep of moving sand or the like from the lower chambers, whereby tolower the foundation raft towards the seabed.

Preferably the units themselves are moved vertically downwards from thefoundation raft within cylinders attached to or forming part of theraft. prior to the founding of the raft.

Preferably further movement of the units within the cylinders iseffected to level the foundation raft after the steps specified.

Two specific embodiments of the invention will now be described by wayof example with reference to the accompanying drawings, in which:

FIGS. 1 to 5 show successive stages in the founding and assembly of afirst marine platform FIG. 6 is a view of a foundation raft (for adifferent maritime platform) in a drydock,

FIG. 7 shows the foundation raft afloat. with a semisubmersiblesupported thereon,

FIG. 8a shows the foundation submersible at sea.

FIG. 8b illustrates the floating stability of the combination.

FIG. 9 shows the foundation raft being lowered to the sea-bed,

FIGS. 10a and 1017 show the founding of the foundation raft on boulderclay and on a soft sea-bed respectively,

FIGS. 11a and 11 show the foundation raft back filled in position,

FIG. vl2 illustrates the foundation raft waiting for a tower structure.

FIGS. l3, l4 and 15 illustrate stages in the construction of the towerstructure,

FIG. 16 shows the tower structure supported on pontoons,

FIG. 17 shows the tower structure floating on its own with an aid of afloation raft,

FIG. 18 illustrates the alignment of the tower structure on thefoundation raft,

FIG. 19 shows the tower structure being winched into position on thefoundation raft, and

FIG. 20 illustrates the completed gravity maritime platform,

FIG. 21 shows a central section of the marine platform nine legs, and

FIGS. 22, 23, and 24 show successive stages in the lowering of thefoundation raft.

In the manufacture of maritime platforms. for example off-shoreplatforms for use in the production of hydrocarbons, it is desirablethat as little as possible of the work be done at sea. This isparticularly so when there is a considerable depth of water at the site.The illustrated embodiments of the present invention have been designedwith this desideratum in mind.

Referring first to FIG. 5 a platform 10 (in this example an off-shoreoil drilling platform) is shown founded on a sub-aqueous bed 12. Theplatform comprises a foundation raft or caisson 14, a tower structure I6and a superstructure 32.

The caisson I4 is of reinforced concrete construction and has anupwardly open conical recess 18 (best seen in FIG. I) and a shaft 20coaxial with the recess. The caisson is hollow and is subdivided into anumber of selectively floodable watertight compartments (not shown).

The tower structure 16 may be a steel fabrication or a composite steeland concrete construction comprising a lower cylindrical section 22. inthis instance circular in cross-section and having a lower part 24 (FIG.4) which is tapered to be a close fit in the upwardly open recess 18 ofthe caisson I4. A projecting peg 25 on the tower structure engages aslot 26 in the side of the recess 18. to ensure correct angularorientation of the tower structure relative to the recess.

The main body of the tower structure I6 is subdivided internally into anumber of water-tight compartraft and semiments 27 which can be floodedselectively. These are employed in the operation of sinking the towerstructure.

The upper end 28 of the tower structure is of wavepenetrableconstruction. That is to say. it presents to the waves a sectionconsisting largely of apertures so that the waves can pass around andthrough the structure, rather than impinge heavily upon it. The stressesin the structure thereby are reduced. In this example, the upper part 28of the tower structure is of lattice construction.

A water-tight central shaft or tube 29 extends throughout the length ofthe tower structure. along the longitudinal axis thereof. This shaftcommunicates with the shaft 20 in the caisson I4, thus providing awatertight shaft giving direct access to the sub-aqueous bed 12 fromabove water level.

A superstructure 32 is disposed on top of the wave penetrable and 28above wave level. The superstructure houses the necessary equipment andservices for the drilling operation.

The method by which the platform is assembled on the sub-aqueous bedwill now be described.

Firstly. with reference to FIG. I, the caisson 14, with its internalcompartments empty (in which condition it is buoyant) is towed to thesite. carrying on its upper surface a semi-submersible 34 which isequipped with power winches. Cables 36 from the winches are attached tothe top of the caisson I4.

On arrival at the site. the internal compartments of the foundation raftare flooded under control so that it sinks in a horizontal attitude.Semi-submersible 34 floats as the caisson l4 sinks, and controls thesinking of the caisson by means of the cables 36.

When the caisson has reached the sub-aqueous bed it is founded in thebed (FIG. 2). The founding operation may be effected conventionally. orby means of the method described and claimed in our copending BritishPatent application 4l9l/7I. to which the reader is referred for furtherdetails. Briefly this method consists of employing a positively buoyantremotely controlled excavating vehicle serviced from thesemi-submersible 34 and which passes down the shaft 20 of the caisson l4and then excavates beneath it whilst moving on the undersurface 38. Thisundersurface is sufficiently flat for the vehicle to move about onwheels or tracks on its roof. The excavated material is removed frombeneath the caisson 14 by suction pipes. As the material is excavatedand removed, leaving a void 40, the caisson 14 slowly sinks until it isfounded sufficiently deeply in the sub-aqueous bed. The void 40 can thenbe back-filled, and the vehicle is finally recovered via the shaft. Thecables 36 are then suitably buoyed for future use in guiding the towersection 22 into the recess 18 of the caisson.

The section 22 is towed to the site with its compartments 27 empty sothat it is buoyant and stable in a horizontal attitude. On arrival atthe site. the compartments 27 of the tower structure are successivelyflooded. one after the other. starting with the compartment in thetapered portion 24, until the tower structure adopts a verticalattitude. Winches 41 are installed on the top of the section 22. Thecables 36 already attached to the caisson I4 are passed through sheaves42 on the lower part of the tower structure and thence to the winches4!. Further controlled flooding of the tower structure now is effected,so that it slowly sinks towards the caisson. The guide cables 36 arewinchedin by the winches 4] so to position correctly the tapered portion24 of the tower structure over the recess 18. Sinking of the towerstructure is continued and finally it comes to rest firmly engaged inthe recess 18. FIG. 5. The correct angular orientation of the towerportion is achieved initially by means of the guide cables 36, andfinally by the peg and recess 25 and 26.

As an alternative to the winches 41. the cables 36 may be passed throughthe sheaves 42 to winches on tugs (not shown).

The rigidity of the connection between the tapered portion 24 of thetower structure and the recess 18 is improved if necessary by pressuregrouting to fill any spaces between the two parts.

The central shaft 20, 29 is at least partially dewatered, so that thewater pressure beneath the caisson 14 is reduced. Then the externalwater pressure further improves the stability of the structure, and itmay be utilised to reduce future settlement by preconsolidating thesub-aqueous bed beneath the structure before it has been commissionedfor its final working purpose.

Finally, the superstructure 32 is towed to the site and winched onto thetop of the assemblycomprising the tower structure and the foundationraft, for example in the manner described in our co-pending Britishpatent application No. 5609/72, to which the reader is referred forfurther details.

The remaining figures and description relate to the founding ofa second,and slightly more elaborate maritime platform, which is founded on thesame principals which have been described above.

FIG. 6 illustrates a submersible concrete foundation raft 101 beingconstructed in a drydock. If required a soft skirt 103 or a more rigidbut downwardly hinged skirt can be clamped round the bottom edge of thefoundation raft before it leaves the drydock. FIG. 7 shows thefoundation raft afloat, with a draft of about 8 metres. The bottomportion of the raft is advantageously launched from the drydock with itsbottom slab and 40% of the walls completed, such that it only draws halfof this amount. The foundation raft is then completed while it isafloat. Temporary support columns 102 may be integral with, or fixed tothe under-surface of the bottom portion when it is afloat insufficiently deep water. Should it be envisaged that the foundation raftwill be founded on boulder clay or rock the soft skirt 103 may beweighted down with sand or concrete.

A purpose made semi-submersible 104 can be constructed on top of thefoundation raft. This semi submersible has a flat deck and a pluralityof hollow pillars 110 leading down to its base.

FIGS. 8a and 8h illustrate the stability of the foundation raft andsemi-submersible when towed in a rough sea. Provided thesemi-submersible is securely lashed to the foundation raft, thecombination should be stable and structurally sound in any waveconditions.

FIG. 9 illustrates the founding of the foundation raft on the sea-bed. Acompresser 105 and a pump 106 (exemplary of many such compressors andpumps) are supplied on the deck of the semi-submersible to adjust thebuoyancy and the internal air pressure of the foundation raft during thesinking operation. The compressor and pump are connected to thefoundation raft by tubes 105a and 106a respectively. Power winches 107are also provided to give fine adjustment to the lower ing procedure,and are connected to the foundation.

raft by lines designated 107a. The semi-submersible I04 is designed tohave a small water plane area when suitably ballasted so that forces inthe lowering equipment are comparatively indifferent to wave movements.

FIGS. 10a and 10!) show the foundation raft on the sea-bed. In the caseof FIG. ]0a the bed is of extremely hard boulder clay with big bouldersand there is no scour or settlement risk, and accordingly the skirts I03are filled with sand or concrete to protect the backfilling underneaththe raft. Three temporary support columns 102 can be used to set up thefoundation raft in a level condition when operated as sand jacks. Asshown in FIG. lla sand or concrete is then admitted to the space beneaththe foundation raft down a central tube to back till the gap between thesea bed and the foundation raft, and thereafter the foundation raftitself can be partly ballasted with sand, for instance in case the raftis later to be used for oil storage. Displaced seawater escapes throughperipheral venting holes. Optionally a weak sand-cement mixture can beused for grouting purposes.

In FIG. 1017 the foundation raft rests on a soft sea bed, and materialfrom the sea bed is withdrawn from beneath the foundation raft by anyconvenient means. Thus the foundation raft is allowed to settle into atrough in the sea bed formed thereunder. As shown in FIG. ill) the spacebeneath the foundation raft can be back filled with sand; or raft may belowered right down to the excavated level without requiring backfilling. Thereafter the foundation raft can be fully flooded.

FIG. 12 illustrates the foundation raft in a founded condition with twofloating buoys 109 indicating its position beneath the surface of thesea.

The construction of the tower structure for the platform is shown inFIGS. 13 to 15. Four units 111 (each unit shown being three deep) areerected vertically and supported by guide ropes 112, to form horizontaltubular members in the completed tower structure. Four units 113 (eachunit shown being three deep) are welded into position between the units111, to form columns (131) in the completed tower structure, and a deck114 is constructed parallel with the units 111. In FIG. 15 the steelworkis completed by fitting diagonals I15 and stressing these to make thetower structure rigid.

In FIG. 16 a completed tower structure, possibly weighing 10,000 tonnes,is launched on two pontoons 116 and 117, and this tower structure can betowed to the site. The pontoon ll6 supports the tower structure throughblocks 119, and between the pontoon 117 and the tower structure there isa floatation raft 121.

As shown in FIG. 17 the pontoons can be sunk from beneath the towerstructure, whereon the tower structure will float under its ownbuoyancy.

As shown in FIG. 18 firstly the floatation raft 121 has been allowed tolift the upper end of the tower structure. and secondly power winches122 on the deck 114 can be attached to the lines 123 which lead up tothe buoys 109 in FIG. 12.

In FIG. 19 the tower structure is shown just above the foundation raft101 and FIG. 20 shows an assembly comprising the tower structure withits feet securely anchored to sockets in the foundation raft. At thisstage the super-structure (not shown) for the platform can be liftedinto position by a crane 125 located on the deck I14.

It will be appreciated that the construction of the foundation raft 101and the tower structure 118 as separate units has fabricationadvantages. Further the founding of the foundation raft at an earlystage in the construction period permits the tower structure to beadjusted in case the local sea bed conditions have caused a variation infoundation level.

The tower structure has eight external legs 131 and a central leg 132.The legs of the tower structure are connected to the foundation raft bymeans ofjoints 135 hereinafter described. The distance between the seabed and the mean sea level may typically be 150 metres and to enable thepressure beneath the foundation raft to respond to the differentialpressures created by the passage of waves passing the deck 114 thecentral leg 132 is hollow, and has a water duct passing up therethrough. The water duct is open to the under sea strata beneath thefoundation raft. and is connected to flap valves 136 and 137 mounted onthe central leg 132 beneath lowest wave trough level. The valves are soarranged that water can escape from the central column whenever thetrough of a wave passes the flaps. Typically the flap valves 136 and 137are metres below the surface of the sea. In this way any build up ofpressure beneath the foundation raft may be reduced.

As shown in FIG. the legs are braced together by bracing members 138 andto give stability and additional strength to the platform as a whole itmay be convenient for parts of the legs 131 to be filled with con crete.

As may be seen from FIG. 21, the base of the illustrated leg 131 has aspigot 141 and a foot plate 147 which is supported by brackets 142 and143. A cross member 144 and diagonal bracing member 138 connect thisfoot to the other feet.

The foundation raft 101 has a socket 145 and there is an annular space146 between the socket 145 and the spigot 141. A rubber seal 148 fitsbetween the edge of the plate 147 (forming a shoulder) and the part ofthe raft 101 surrounding the socket 145.

It will be apprecaited that each of the legs may have a spigotarrangement. and that the foundation raft has a socket for each spigot.Alternatively the spigots may be formed on the raft and the sockets mayextend upwardly into the legs.

In use the spigot 141 beneath the tower structure is lowered into thesocket 145 as the tower structure is sunk onto the foundation raft. Whenthe tower structure feet are firmly founded in the foundation raft, therubber seal 148 backed by the edge shoulder is gripped between the plate147, and the top slab of the raft. and effectively seals the spaceunderneath the foot plate from external water pressure. The annulus canbe pumped out. and air at atmospheric pressure can be admitted to thespace through a drain valve 149. In this way the pressure beneath theplate 147 is greatly reduced as compared to the sea water pressure onthe upper surface of that plate. and the tower structure may be held incontact with the foundation raft 101 by the full external hydrostaticpressure on the foot plate 147.

Subsequently the space between the spigot and the socket can be groutedfrom within the spigot. providing an additional tension connectionbetween tower and raft.

If at some future date it is desirable to dismantle the platform againit would be natural to bolt the tower to the raft by means of bolts 150after the seal 148 had been engaged and the socket dewatered. Todismantle the platforms the bolts can be released and the legs releasedfrom the spigots which remain fixed to this foundation raft.

Details of the founding of this foundation raft as shown in FIGS. 10aand 110 will now be described in more detail.

As shown in FIG. 22. the foundation raft 101 is floating in water. Thefoundation raft has generally horizontal upper and lower slabs 212 and212A respectively. and generally vertical webs 214 between the slabs.

Near the periphery of the raft there are three cylinders 215 eachcontaining a piston 216. For convenience only one such piston cylinderarrangement is shown. The piston 216 is hollow and has vertically spacedupper and lower chambers 217 and 218 respectively. At the base of thechamber 218 there is a generally horizontal plate 219 supported oncables 221. The plate 219 closes the base of, and is movable verticallywithin. the lower chamber 218.

As shown in FIG. 23, the piston 216. which was earlier completelyretracted into the recess 215. has now been lowered downwardly and issupported by cables 222. This is the situation when the foundation rafthas reached its intended site.

A mixture of sand and water is pumped into the space above the piston216 and also into the lower chamber 218. In this configuration thefoundation raft is lowered towards the sea bed.

Initially the piston 216 penetrates a distance a' into the strata of thesea bed. This penetration can be effected by suitable ballasting of thefoundation raft or by high pressure water jetting underneath the lowerplate 219.

If further penetration into the sea bed is required this can be achievedby shifting sand from chamber 218 to chamber 217 by means of highpressure water jets. The walls of the piston will penetrate the distanceh into the sea bed in accordance to the amount of sand shifted. leavingplate 219 at its original level.

By excavation under the foundation raft as a whole. and shifting sand asrequired from chambers 218 to 217 (arrow X) it is possible to lower theraft to a level where plate 219 is resting against the dividing floor220 and all the sand has been removed from chamber 218.

The lowering process can thereafter be continued by jetting sand out ofthe space (225) above the piston. When the space 225 has been reduced tothe same depth as the working space underneath the raft the ex cavationmay be stopped. If thereafter further sand is jetted from the space 225(arrow Y) the raft will settle down to a lower level.

We claim:

1. A maritime assembly for disposal on a sub-aqueous bed. comprising afoundation raft to be founded on the bed. and a tower structure toproject upwardly therefrom when founded. in which one of the foundationraft and the tower structure has a socket portion and the other has acorresponding spigot portion for engagememt therewith, and there are onthe base of the foundation raft three units. each having two verticallyspaced chambers. the lower of which chambers contains particulatematerial and there are means to remove particulate material from thelower chambers. whereby the raft can be lowered towards the seabed bydisplacement of the particulate material.

2. An assembly as claimed in claim 1, in which each unit has a platearranged at the foot of the lower chamber to bear on the seabed, whichplate is connected to the unit by cables, and is slideable within thelower chamber.

3. An assembly as claimed in claim 2, in which the units themselves aremoveable as pistons within cylinders attached to or forming part of thebase of the raft. and the height of the pistons can be adjusted bymovement of particulate material from the aforesaid cylinders.

4. An assembly as claimed in claim 3 wherein the pistons are suspendedwithin the cylinders to prevent them from dropping out,

5. A method of founding a maritime assembly, which includes the steps offounding a foundation raft on a sub-aqueous bed, and sinking a towerstructure onto the foundation raft, in which spigot and socket portionson the foundation raft and the tower structure are engaged to secure thetower structure to the raft, and in which three units are located on thebase of the foundation raft, each of which units have two verticallyspaced chambers, the lower of which chambers contains particulatematerial and is separated from the seabed; including the step ofremoving particulate material from the lower chambers, whereby to lowerthe foundation raft towards the seabed.

6. A method as claimed in claim 5, in which the units themselves aremoved vertically downwards from the foundation raft within cylindersattached to or forming part of the raft prior to the founding of theraft.

7. A method as claimed in claim 5, in which further movement of theunits within the cylinders is effected to level the foundation raftafter the steps specified.

8. A method of founding a maritime assembly on a seabed, comprising thesteps of locating a unit on the base of the assembly which unit hasdownwardly extending sidewalls forming an open ended downwardly facingchamber having sand therein which chamber is separated from the seabedby movable closure means arranged across the open end of the chamber andmovable thereinto to reduce the capacity of the chamber, sinking theassembly onto the seabed torest on the unit, and removing sand from thechamber to reduce its capacity and to lower the assembly toward theseabed as sand supporting the assembly is removed from the chamberthereby increasing penetration of the sidewalls of the unit into theseabed.

9. A unit for attachment to the base of a foundation raft to be foundedon a subaqueous bed, and having downwardly extending sidewalls formingan open ended downwardly facing chamber adapted to contain particulatematerial, movable closure means therefor disposed across the open end ofthe chamber constructed and arranged to be movable thereinto to reducethe capacity of the chamber and to prevent particulate materialcontained therein from falling out therefrom, means movably supportingsaid movable closure means, and water jetting means to removeparticulate material therefrom.

10. A unit as claimed in claim 9 containing particulate material.

ll. A unit as claimed in claim 9 in which the chamber has a uniformhorizontal cross section, and the closure means is a horizontal plateretained within the chamber and arranged to move up or down therein.

12. A unit as claimed in claim 9 in which the aforesaid chamber is alower chamber, and there is an upper chamber arranged to receiveparticulate material removed from the lower chamber.

13. The unit as claimed in claim 9 in which the chamber is itselfmovable as a piston within a downwardly facing cylinder for attachmentto the base of the raft. and the height of the piston can be adjustedupwardly by removal of particulate material from the cylinder.

14. The unit as claimed in claim 13 including means for suspending thepiston within the cylinder to prevent it from dropping out.

15. A foundation raft for a maritime structure having at least threeunits of the kind set forth in claim 10 attached to its base, wherebythe raft can be lowered towards the sea bed. by the displacement ofparticulate material chambers.

16. A method of founding a foundation raft for a maritime structure inwhich at least three units of the kind set forth in claim 10 are locatedon the base of the foundation raft, and including the step of removingparticulate material from the chambers to reduce their capacity and tolower the foundation raft towards the seabed as particulate materialsupporting the foundation raft is removed from the chambers, therebyincreasing penetration of the sidewalls of the units into the subaqueousbed.

17. The method as claimed in claim 16 including the step of controllingremoval of particulate material from said units to level the foundationraft after its initial founding.

1. A maritime assembly for disposal on a sub-aqueous bed, comprising afoundation raft to be founded on the bed, and a tower structure toproject upwardly therefrom when founded, in which one of the foundationraft and the tower structure has a socket portion and the other has acorresponding spigot portion for engagememt therewith, and there are onthe base of the foundation raft three units, each having two verticallyspaced chambers, the lower of which chambers contains particulatematerial and there are means to remove particulate material from thelower chambers, whereby the raft can be lowered towards the seabed bydisplacement of the particulate material.
 2. An assembly as claimed inclaim 1, in which each unit has a plate arranged at the foot of thelower chamber to bear on the seabed, which plate is connected to theunit bY cables, and is slideable within the lower chamber.
 3. Anassembly as claimed in claim 2, in which the units themselves aremoveable as pistons within cylinders attached to or forming part of thebase of the raft, and the height of the pistons can be adjusted bymovement of particulate material from the aforesaid cylinders.
 4. Anassembly as claimed in claim 3 wherein the pistons are suspended withinthe cylinders to prevent them from dropping out.
 5. A method of foundinga maritime assembly, which includes the steps of founding a foundationraft on a sub-aqueous bed, and sinking a tower structure onto thefoundation raft, in which spigot and socket portions on the foundationraft and the tower structure are engaged to secure the tower structureto the raft, and in which three units are located on the base of thefoundation raft, each of which units have two vertically spacedchambers, the lower of which chambers contains particulate material andis separated from the seabed; including the step of removing particulatematerial from the lower chambers, whereby to lower the foundation rafttowards the seabed.
 6. A method as claimed in claim 5, in which theunits themselves are moved vertically downwards from the foundation raftwithin cylinders attached to or forming part of the raft prior to thefounding of the raft.
 7. A method as claimed in claim 5, in whichfurther movement of the units within the cylinders is effected to levelthe foundation raft after the steps specified.
 8. A method of founding amaritime assembly on a seabed, comprising the steps of locating a uniton the base of the assembly which unit has downwardly extendingsidewalls forming an open ended downwardly facing chamber having sandtherein which chamber is separated from the seabed by movable closuremeans arranged across the open end of the chamber and movable thereintoto reduce the capacity of the chamber, sinking the assembly onto theseabed to rest on the unit, and removing sand from the chamber to reduceits capacity and to lower the assembly toward the seabed as sandsupporting the assembly is removed from the chamber thereby increasingpenetration of the sidewalls of the unit into the seabed.
 9. A unit forattachment to the base of a foundation raft to be founded on asubaqueous bed, and having downwardly extending sidewalls forming anopen ended downwardly facing chamber adapted to contain particulatematerial, movable closure means therefor disposed across the open end ofthe chamber constructed and arranged to be movable thereinto to reducethe capacity of the chamber and to prevent particulate materialcontained therein from falling out therefrom, means movably supportingsaid movable closure means, and water jetting means to removeparticulate material therefrom.
 10. A unit as claimed in claim 9containing particulate material.
 11. A unit as claimed in claim 9 inwhich the chamber has a uniform horizontal cross section, and theclosure means is a horizontal plate retained within the chamber andarranged to move up or down therein.
 12. A unit as claimed in claim 9 inwhich the aforesaid chamber is a lower chamber, and there is an upperchamber arranged to receive particulate material removed from the lowerchamber.
 13. The unit as claimed in claim 9 in which the chamber isitself movable as a piston within a downwardly facing cylinder forattachment to the base of the raft, and the height of the piston can beadjusted upwardly by removal of particulate material from the cylinder.14. The unit as claimed in claim 13 including means for suspending thepiston within the cylinder to prevent it from dropping out.
 15. Afoundation raft for a maritime structure having at least three units ofthe kind set forth in claim 10 attached to its base, whereby the raftcan be lowered towards the sea bed, by the displacement of particulatematerial chambers.
 16. A method of founding a foundation raft for amaritime structure in which at least three units of thE kind set forthin claim 10 are located on the base of the foundation raft, andincluding the step of removing particulate material from the chambers toreduce their capacity and to lower the foundation raft towards theseabed as particulate material supporting the foundation raft is removedfrom the chambers, thereby increasing penetration of the sidewalls ofthe units into the subaqueous bed.
 17. The method as claimed in claim 16including the step of controlling removal of particulate material fromsaid units to level the foundation raft after its initial founding.